The REAL COD Removal by a Methanator

Methanators perform their best when the COD removal is consistently above 95%. This not only reduces many of the organic acids, including acetic, but keeps the bugs in the reactor. The difference between Methanators that run 90% removal and 95% removal is much more than just 5%. Buried in that 5% is the difference between growing bugs and buying bugs.

So you look at your COD removal spreadsheet and it says you are at 95%. But how accurate is that? The composite sampler is a steel pot that collects a small sample over the course of a day. The pot also happens to collect small amounts of bacteria and remains fairly warm. Those bacteria can and do continue to work on any remaining COD. Here’s an example of the difference between the effluent COD grab and composite samples. It can be up to a few hundred mg/L.

The best way to get accurate effluent COD values is to just discontinue the effluent composite and only use a grab. It would initially appear that the composite sample would be better but in this case the grab is a better indicator of what the Methanator is actually doing. In a way, the grab COD is a partial composite sample since the hydraulic retention time in the reactor is almost half a day. The difference in COD could be a hidden organic acid load not being accounted for that is affecting fermentation.